On-chip and off-chip communication has emerged as a critical issue for sustaining performance growth for the demanding, data-intensive applications for which many chips are needed. Computational bandwidth scales linearly with the growing number of transistors, but the rate at which data can be communicated across a chip using top-level metal wires is increasing at a much slower pace. In addition, the rate at which data can be communicated off-chip through pins located along the chip edge is also growing more slowly than compute bandwidth, and the energy cost of on-chip and off-chip communication significantly limits the achievable bandwidth.
Optical interconnects including optical fibers or waveguides have been proposed as an alternative to wires used in on-chip and off-chip communications. For example, a single fiber optic cable can carry terabits per second of digital information encoded in different wavelengths of light called optical signals with a capacity ranging from about 4×104 to about 5×104 times greater than transmitting the same information using wires (cf. 5 GHz Pentium with 200 THz optical signal at 1.5 micron wavelength). Because of the increasing interest in transmitting data in optical signals, much interest is now being paid to small scale light sources that can be modulated to generate optical signals. The light-emitting diode (“LED”) is a low cost light source that can be modulated to encode data in optical signals. Common LEDs include a depletion layer, and in some cases may include a thin undoped or intrinsic semiconductor layer, sandwiched between a p-type semiconductor layer and an n-type semiconductor layer (see e.g., S. Sze, Ch 12.3.2 of Physics of Semiconductor Devices, 2nd Ed., Wiley, New York, 1981). Electrodes are attached to the p-type layer and the n-type layer. When no bias is applied to an LED, the depletion layer has a relatively low concentration of electrons in a corresponding conduction band and a relatively low concentration of vacant electronic states called “holes” in a corresponding valence band and substantially no light is emitted. The electrons and holes are called “carriers.” In contrast, when a forward-bias operating voltage is applied across the layers, electrons are injected into the conduction band of the depletion layer, while holes are injected into the valence band of the depletion layer creating excess carriers. The electrons in the conduction band spontaneously recombine with holes in the valence band in a radiative process called “electron-hole recombination” or “recombination.” When electrons and holes recombine, photons of light are emitted with a particular wavelength. As long as an appropriate operating voltage is applied in the same forward-bias direction, nonequilibrium carrier population is maintained within the depletion layer and electrons spontaneously recombine with holes, emitting light of a particular wavelength in nearly all directions. When the bias is removed, excess carriers remaining in the depletion layer can recombine or the built-in electric field of the p-n junction can sweep the excess carriers from the depletion layer, and radiative recombination stops. The radiative recombination fall-off time is determined by the excess carrier lifetime or by the time it takes the excess carriers to drift through the depletion layer. Typically, in high-quality materials, the excess carrier lifetime is long. In some cases, therefore, excess carriers continue recombining for a period of time after the voltage is removed. Thus, the emitted optical signal may not decrease substantially for a period of time after the voltage is turned off or becomes low.
A data-encoded optical signal generated by modulating an LED is ideally composed of distinguishable high and low intensities. For example, high and low operating voltage pulses corresponding to the bits “1” and “0” can be applied to an LED to encode the same information in high and low intensities of light emitted from the LED. High intensity light emitted from an LED for a period of time can represent the bit “1,” and low intensity or no light emitted from the LED for a period of time can represent the bit “0.” In practice, however, when the operating voltage is modulated at high speeds, such as about 50 GHz, the high and low intensities of the optical signal may be indistinguishable because the LEDs can continue to emit light between applications of the operating voltage.
FIG. 1 shows a first plot 102 of a modulated, forward-bias operating voltage applied to an LED versus time. FIG. 1 also shows a second plot 104 of the corresponding intensity of an optical signal emitted from the LED versus time. In plots 102 and 104, horizontal axes 106 and 108 represent time, vertical axis 110 represents the magnitude of the forward-bias operating voltage, and vertical axis 112 represents intensity of light emitted from the LED. Rectangles 114-116 represent the magnitude and duration of voltage pulses composing the modulated, forward bias, operating voltage applied to an LED, where between each pulse, the voltage is turned off. The plots 102 and 104 reveal that light is emitted from the LED with relatively constant and continuous intensities 118-120 during the time periods when the pulses 114-116 are applied. However, the plot 104 also reveals that during the time periods between pulses 114-116, the LED continues to emit light with an intensity that slowly drops off but not completely before the next pulse is applied. In particular, curved portions 122-124 represent slow relative intensity drop offs after the pulses 114-116 are turned off.
The slow relative drop off in intensity is the result of excess electrons remaining in the conduction band and holes remaining in the valence band of the depletion layer when the voltage is turned off. These electrons and holes continue to recombine in the absence of an operating voltage. In addition, because of the high modulation speed, a subsequent operating voltage pulse is applied before the excess electrons and holes have had a chance to complete recombination. Thus, high and low intensity portions of an optical signal may be indistinguishable. Accordingly, LEDs that exhibit rapid output light intensity drop off during high speed modulation are desired.